This invention relates to an engine control method whereby in response to the operating conditions of an engine, a microcomputer computes the optimum value to be controlled for an engine component part to be controlled (controlled system), such as an ignition system and the engine is controlled in accordance with the result of the computation.
Generally, known methods for controlling the ignition system of an engine through a microcomputer are so designed that the energization period of the ignition coil of the ignition system (or the energization starting timing) and the time to effect the ignition (or the energization stopping time) are each computed once in terms of crank angle degrees so as to control the energization period and the ignition timing of the ignition system for a plurality of times, and the computation of ignition timing is never effected anew during the time that the ignition coil is being energized. As a result, when the engine rotational speed changes abruptly such as during the acceleration or deceleration period of the engine, despite the need to set the ignition timing to a computed value based on the later engine data, the desired computation based on the change in the engine speed cannot be effected with the resulting disadvantage that it is impossible to produce the satisfactory power output as well as the satisfactory exhaust gas composition of the engine.
It has also been found that even though that computation of energization period and ignition timing is performed many times for every engine revolution and a new control value for the ignition timing is computed and set in the ignition system while the ignition coil is being energized so as to control the ignition timing, the ignition coil is already being energized in accordance with the computed value determining the energization period with the result that if the newly computed value of the ignition timing differs greatly from the previously computed value, the actual energization period will vary greatly from the previous energization period and consequently such abnormal combustions as engine misfiring and the like will be caused, if the ignition coil fails to store a sufficient electrical ignition energy.